1. Technical Field
The present invention relates to a condenser stereomicrophone adoptive to plug-in power.
2. Background Art
Condenser stereomicrophones have microphone capsules or electroacoustic transducers that have significantly high output impedances. The output impedances are reduced through impedance converters. An impedance converter is usually equipped with a field-effect transistor (FET). Thus, a condenser microphone requires a power source for the operation of the FET in the impedance converter.
A microphone capsule of a condenser microphone is an electroacoustic transducer that consists of a condenser having a diaphragm and a fixed electrode which face each other and converts the vibration of the diaphragm by acoustic waves into electrical signal. A microphone capsule is also referred to as a microphone module. The term “microphone capsule” will be used in the specification. The microphone capsule and the impedance converter constitute a condenser microphone unit. The condenser microphone unit is accommodated in a microphone case. A microphone includes a microphone unit, a front mesh, a circuit board, and other components.
A professional-use condenser microphone is connected to an external phantom power supply via a connector, for example three-pin XLR connector. A phantom power supply is aimed to supply a polarization voltage to the condenser microphone, and can also be used as a power source of the impedance converter.
The use of the phantom power supply is unintended for condenser microphones connected to conventional consumer-use IC recorders and video recorders (which, hereinafter, are collectively referred to as recording devices). A mechanism has been employed in microphones for receiving electrical power from a power source in a recording device through a plug of the microphone in connection with a microphone jack in the recording device. Such a mechanism is referred to as plug-in power.
Stereomicrophones record and reproduce three-dimensional sounds by recording left and right channels separately. A known stereomicrophone has a single housing accommodating microphone capsules for the left and right channels. A mid/side (MS) stereomicrophone with narrow directionality has a mid capsule and side capsules, the mid capsule having a directional axis extending along the main axis of the microphone body, and the side capsules having directional axes in the direction orthogonal to the main axis.
The side capsules of a conventional MS stereomicrophone have bidirectionality. Unfortunately, bidirectional microphone capsules are expensive. Thus, the inventors have proposed a narrow-directionality stereomicrophone having a mid capsule and two unidirectional microphone capsules (refer to PTL 1, Japanese Unexamined Patent Application Publication No. 2012-178628). According to PTL 1, the two unidirectional microphone capsules are arranged such that their directional axes are symmetrical about the main axis of the microphone body.
PTL 1 provides the narrow-directionality, which is the original target, of the stereomicrophone, but does not consider plug-in power. Plug-in power can be established in the invention according to PTL 1 only by providing a source terminal VDD in addition to the left and right signal output terminals, as illustrated in the drawings in PTL 1.
For example, the right or left output terminal may be connected to the source terminal VDD in the circuit illustrated in the circuit diagram in PTL 1 to establish plug-in power. Such a connection will cause an imbalance in impedance between the right and left output terminals and thus a difference in intensity between the left and right audio outputs, failing to produce satisfactory stereo output. The output signals from the right and left output terminals connected to the source terminal VDD are identical. In other words, the overall output signal obtained through the connection is a monaural signal.
The output terminals and the source terminal VDD may be connected with resistors in the circuit illustrated in the circuit diagram of PTL 1. In such a case, the resistor connecting the right output terminal and the source terminal VDD is set to be the same as the resistor connecting the left output terminal and the source terminal VDD. In this way, the plug-in power is established through the electrical power supplied from the recording device to the condenser microphone unit through the source terminal VDD connected to the right and left output terminals via the resistors. In such a configuration, however, the resistors restrict the current supplied to the source terminal VDD, and thus an insufficient current is supplied to the active devices, such as an FET, in the impedance conversion circuit. The configuration described above results in low left and right output levels.
A zoom microphone is also disclosed that can support plug-in power and can be used as a stereomicrophone (refer to PTL 2, Japanese Unexamined Patent Application Publication No. 2001-28795). The zoom microphone according to PTL 2 includes a central microphone, left and right channel microphones, resistors R1 and R2 connected in series with the left and right channel microphones, and a selector switch. The selector switch switches the recording mode. There are two recording modes: a zoom mode in which the central microphone is connected to connection points of the resistors R1 and R2, and a stereo mode in which the central microphone is disabled by grounding the connection points of the resistors R1 and R2.
The zoom microphone according to PTL 2 is adaptive to plug-in power. The zoom mode combines the output of three microphones, i.e., the central microphone and the left and right channel microphones. Thus, the zoom microphone according to PTL 2 cannot output left and right stereo signals. The zoom microphone according to PTL 2 collects sounds over a wide angle in the stereo mode. Thus, the zoom microphone according to PTL 2 cannot be used as a narrow-directionality stereomicrophone with plug-in power.